Primer on corona and arcing

03/01/2003

Sponsored by

Dan Ninedorf, Specialized Camera Sales & Services LLC

Insulating materials that are failing can cause flash-overs and outages. Such imminent failures can be indicated by corona, which shows the presence of an intense electrical field. For a more detailed breakdown of coronas and arcing, EL&P talked with Dan Ninedorf.

What is electrical corona?

Corona is the ionization of the nitrogen in the air, caused by an intense electrical field. Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage, is invisible during the day, and requires darkness to see. Arcing starts at a voltage and stops at a voltage about 50 percent lower and is visible to the naked eye day or night, if the gap is large enough (about 5/8" at 3500 volts).

What are the indications of electrical corona?

A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.

What damage does corona do?

The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-overs. The corona is the only indication. Defects in insulating materials that create an intense electrical field can, over time, result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

How long does corona require to create visible damage?

Example: In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator. At the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was gray.

What is flash-over and arcing?

Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not have the current available to sustain an arc. An arc can have the grid fault current behind it and sustain until the voltage decreases below 50 percent or until a protective device opens. Flash-over can also occur due to induced voltages in unbonded (loose bolts, washers, etc.) power pole or substation hardware, this can create RFI/TVI or radio/tv interference. Arcing can begin at 5 volts on a printed circuit board or as the insulation increases it may require 80kVAC to create flash-over on a good pin and cap insulator.

What causes flash-over?

Flash-over causes are not always easily explained. They can be cumulative, and usually result in an outage and destruction. The first flash-over components are available voltage and the configuration of the energized parts. Corona may be present in many areas where the flash-over occurs. Flash-over can be excited by stepping stone defects in the insulating path.

What are causes of insulator failure?

Electrical field intensity producing corona on contaminated areas: water droplets, icicles, improper corona rings. This corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid expansion causing fracture or rupture of the material. It has also been determined that electrical stress can be a real culprit in EHV insulation, both porcelain and non-ceramic.

How do I test insulators?

Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to contain an insulator that may shatter. Due to steam build-up from moisture in a cavity, arcing produces intense heat. An AM radio is a good RFI/arcing detection device. A bucket truck AC dielectric test set (130KV) is a good test set for most pin and cap type insulators. Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-overs that are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or other transients happen. Stop when the line is de-energized or dropped below 50 percent of arc ignition voltage.

What voltage does arcing extinguish at?

It depends, but most of the time about 50 percent of the voltage it started at. Remember this when troubleshooting a radio interference problem. If the radio noise is there, turn the line off. If it goes away, turn the line on again. If the noise is not present, then it is probably arcing caused by capacitor switching or transients. If the noise is there when the line is turned back on, then it is probably arcing and this can be visible a mile away with a daytime corona camera.

What is a good, readily available RFI/TVI noise detector?

The AM radio in your car or truck tuned where there is no radio station and with the volume adjusted for comfort will do just fine. Drive into the radio interference complaint area and listen for the loudest pole. Some noises can be outside the radio reception and will require other detectors.

How does this affect the reliability of our electrical system?

Corona is a symptom; it may be present for years before the component finally fails. Corona can be an indication or the catalyst of the chemical soup that permeates insulator bonding cements preparing them for internal flash-over.

Ninedorf is with Specialized Camera Sales & Services LLC, the CoroCAM corona camera distributor. Contact him for technical, camera and other test set questions at cameras@maqs.net or by telephone 888-322-9368. This article was reviewed for technical accuracy by Jim Booker of J.R. Booker Consulting. Contact Jim with questions at jbooker@greenapple.com or telephone 740-862-3113. Ninedorf and Booker present the Corona Technology Course. More information can be found at www.corona-technology-course.com. Ninedorf's company website is www.specialcamera.com.